Parents try to be fair, but children pick up on subtle differences in the way they are treated.

In a study appearing in the journal Child Development, researchers led by Jennifer Jenkins, a professor of human development and applied psychology at the University of Toronto, report on the wide-ranging effects that playing favorites, known as differential parenting, can have on not just individual siblings but also on the behavior and mental health of all family members.

When parents provide more positive feedback and encouragement to one child while sending primarily negative comments to another, it’s no surprise that the negatively targeted child may develop more behavior problems and have a more difficult relationship with his parents. But Jenkins and her colleagues were interested in exploring how differential parenting affects all siblings in a family and in understanding some of the factors that might make such differential treatment more likely.

The researchers focused on nearly 400 Canadian families, each of which had at most four children. They asked mothers about their children’s positive and negative behaviors and went to some of the families’ homes to observe parent-child interactions — for example, how the children played without toys and how mothers taught their children to make a pattern based on a photo, and the way mothers told their children a story. When the youngest children were at least 18 months old, the researchers measured the youngsters’ aggression, attention and emotional problems and rated their relationships with siblings and parents.

Overall, the negatively treated children tended to show more attention and emotional problems than their more positively treated siblings by the end of the four-year study, but all children showed higher rates of these problems compared with when the trial began. That, says Jenkins, was a surprise since previous work had only highlighted the effect of differential parenting on the targeted children.

“We would have thought that, on the basis of previous research, it would just be the disfavored children who are having problems, but that’s not the case,” says Jenkins. “Sometimes moms are very similar with their kids, and sometimes they’re very different with their kids. And when they advantage some and disadvantage others, it looks like it’s a problem for all of the kids in terms of their mental health.”

Since parents rarely set out to treat their children differently, the scientists decided to explore which factors promoted differential parenting; they figured that a mother’s stress due to economic or personal experiences might have the greatest impact on the way she treats her children, so they also collected data on the mothers’ education levels, depression and history of physical and sexual abuse. They also considered family dynamics such as whether the mother was a single parent raising her family and the safety of the home environment.

Mothers who came from unstable family backgrounds were more likely to treat each of their children differently than mothers who had privileged upbringings, and the more external factors a mother faced, like being a single parent or struggling with depression, the more difficult it was for her to treat her children equally.

That makes sense, Jenkins says, since a mother who is stressed may have less patience with the child who has more problems. But as a result, all of her children may experience more behavior problems since children tend to internalize their parents’ reactions. Children who perceive the differential treatment as unfair, even if they are treated more positively, may also act out and develop attention or emotional problems as they grow older, possibly as a way to empathize with their siblings or in protest of the injustice of the favoritism. “It creates a sense of the kids feeling uneasy, or [gives them a] sense of unfairness,” says Jenkins. “In other studies, we’ve shown that the amount of differential parenting is related to the quality of sibling relationships, so when a kid is favored or disfavored, [he or she] is fed up with the sibling, getting on less well with that sibling, and that also feeds into the general dynamic of the family.”

Addressing some of the factors that may fuel parents to play favorites, like financial strain, could help alleviate some of the wide-ranging effects on families. “We really have to start supporting parents who are socially disadvantaged and make sure they have access to money so they aren’t worried about their kids all of the time, and access to good childcare so that they’re not worried about kids being on their own when they’re working,” says Jenkins. “These things are eating into the way parents can parent.”

Even when parent-child relationships are stressed in this way, communicating well can reduce some of the negative consequences. In some families, certain siblings need more attention or support than others, and parents should discuss with their kids why they are approaching siblings differently to avoid any misunderstanding. Children “don’t mind that parents treat them differently,” Jenkins says. “They only mind when they see that differential treatment as unfair, and that comes about when things aren’t explained to children.”